Measuring and controlling the thickness of a conductive coating on an optical fiber

ABSTRACT

A method of for manufacturing a coated optical fiber includes depositing a conductive coating on the optical fiber and measuring a value of conductance of that coating. Featured within the manufacturing method is a method for measuring the thickness of the conductive coating on an insulator, e.g., carbon on an optical fiber, including the following steps. An electromagnetic field is established by an input signal. The conductively coated insulator is moved through the energized electromagnetic field. The conductive coating on the insulator is oriented with respect to the electric field so that their interaction increases transmission loss from input to output. An output signal is extracted from the electromagnetic field. From changes in the output signal with respect to a predetermined standard, or reference, the conductance and the thickness of the conductive coating are determined. From the measured thickness of the coating, a control signal is generated for dynamically controlling one or more of the process parameters for depositing the coating on the insulator from a precursor gas. The coated insulator continuously moves through the apparatus without any physical contact. No interruption of the production process occurs.

This invention relates to a method used in making a coated elongatedinsulator, such as an optical fiber.

BACKGROUND OF THE INVENTION

Performance characteristics of optical fibers can be degraded byenvironmental elements. For example, the interaction of water with thesurface of a silica fiber produces surface modifications which canreduce the strength of the fiber. Also over a period of time, hydrogencan diffuse into an optical fiber and increase the optical loss in asignal carried by that optical fiber.

In order to prevent such interactions, a coating can be applied to thefiber for preventing deleterious environmental elements from interactingwith the fiber. Ideally such a coating acts as an impenetrable hermeticbarrier between the fiber and the environment. One such coating, e.g., acarbon coating, is applied under stable ambient conditions to the outersurface of a silica cladding of the fiber by inducing decomposition of asuitable carbon containing organic precursor gas, e.g., acetylene, toform a thin carbon film on the fiber surface, as described by F. V.DiMarcello et al., in a U.S. patent application, Ser. No. 098253, filedSept. 18, 1987, now abandoned. For optimum results, the carbon coatingmust be applied at a particular thickness within close tolerances. Ifthe coating is too thin, it does not sufficiently limit the penetrationof the undesirable environmental elements, such as water and hydrogen.On the other hand, if it is too thick, fiber strength can be reduced bymicrocracks which can form in the carbon coating when the fiber is underhigh tensile force.

A need therefore has arisen for a dynamic method to measure and tocontrol the thickness of the coating being applied to the fiber. Themethod should allow continuous production of the fiber and avoid anydirect contact with the fiber. Any interruption of the continuousdrawing process is intolerable. Physical contact with an unjacketedoptical fiber can damage the surface and reduce the tensile strength ofthe fiber. In the prior art, however, thickness of a coating has beenmeasured by static off-line metrology, e.g., by electron microscopy orby calculation based on a direct current measurement of resistance in anelectrical circuit. These prior art methods require either theinterruption of the drawing process, physical contact with theunjacketed optical fiber, or both.

SUMMARY OF THE INVENTION

These and other problems are solved by a new method for making anoptical fiber. This manufacturing method includes the steps of:depositing a coating on the moving optical fiber and by means of anon-contact electromagnetic field measuring a value of effective radiofrequency conductance of the coating for dynamically controlling thethickness of that coating being deposited on the optical fiber.

Featured within the manufacturing method is a method for measuring thethickness of a conductive coating on an insulator, e.g., carbon on anoptical fiber. Measuring the thickness of the coating includes thefollowing steps. A radio frequency electromagnetic field is establishedby an input signal. The coated insulator is moved through the energizedelectromagnetic field at a position where the electric field issufficiently strong to create a useful output signal. The coating on theinsulator is oriented with respect to the electric field or a componentthereof so that its interaction with the conductive coating increasestransmission loss from input to output. An output signal is extractedfrom the electromagnetic field at a point where the output signal can bedetected. Such point is remote from the fiber location. Extraction canbe accomplished by either an electric field probe or a magnetic fieldprobe. From changes in the output signal with respect to a predeterminedstandard, the effective radio frequency conductance of the conductivecoating is determined. Thickness of the coating is determined from theconductance data.

The following advantages are achieved by the foregoing method. Thecoated insulator continuously moves through the apparatus for measuringwithout any physical contact. No interruption of the production processoccurs. From the thickness determination, signals are generated fordynamically controlling the coating process to maintain a desiredthickness tolerance.

The general principles here stated can be applied over a wide range ofradio frequencies, typically from about 10 MHz to 150 GHz, and can beapplied to a wide range of coating configurations by appropriatelyselecting a frequency range and equipment that is compatible with theselected frequency range.

BRIEF DESCRIPTION OF THE DRAWING

A better understanding of the invention may be derived by reading thesubsequent detailed description thereof with reference to the attacheddrawing wherein:

FIG. 1 is a schematic diagram of an arrangement for drawing an opticalfiber and of apparatus for making measurements and controlling thedrawing operation and the coating process;

FIG. 2 is a diagram of a section of optical fiber without its polymericjacketing;

FIG. 3 is a diagram of a section of optical fiber coated with aconductive carbon coating;

FIG. 4 is a schematic diagram of an arrangement for measuring thethickness of the conductive carbon coating on the optical fiber;

FIG. 5 is a perspective drawing of the vectors of an incidentelectromagnetic wave and its resulting reflected electromagnetic waveproduced at a short circuit termination;

FIG. 6 is a series of curves representing output energy versus frequencyfor different measurements taken on an optical fiber;

FIG. 7 is a schematic diagram of another arrangement for measuring thethickness of the conductive coating on the optical fiber;

FIG. 8 is a schematic diagram of a third arrangement for measuring thethickness of the conductive coating on the optical fiber;

FIG. 9 is a schematic diagram of a fourth arrangement for measuring thethickness of the conductive coating on the optical fiber;

FIG. 10 is a transmitted power loss versus conductance, or thickness,characteristic for an exemplary coated fiber;

FIG. 11 is a schematic diagram of a fifth arrangement for measuring thethickness of the conductive coating on the optical fiber

FIG. 12 shows a cross-section view taken from the arrangement of FIG.11;

FIG. 13 is a schematic diagram of a sixth arrangement for measuring thethickness of the conductive coating on the optical fiber;

FIG. 14 shows a cross-section view taken from the arrangement of FIG.13;

FIG. 15 is a schematic diagram of a seventh arrangement for measuringthe thickness of the conductive coating on the optical fiber; and

FIG. 16 is a schematic diagram of an eighth arrangement for measuringthe thickness of a conductive coating on the optical fiber.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown a diagram of an exemplaryequipment for drawing an optical fiber 20 from a preform 24. The preformmay include silica glass with predetermined dopants which will form anoptical fiber having a low loss optical core section that is surroundedby a cladding section. The core and cladding sections have differentindices of refraction so that light transmitted axially along the coreis retained within the core because of internal reflections and/orconfinement which occur due to the stratification of the refractiveindicies.

A furnace 25 surrounds at least the lower end of the preform 24 andheats that end to its melting or softening temperature. The fiber 20 isdrawn from the end of the preform 24 at a velocity, or rate, that isknown to produce the fiber 20 with a predetermined diameter. Generallythe fiber is drawn from the preform at a controlled steady temperatureand velocity. Other combinations of temperature and velocity, or rate,of the draw can be used for producing fibers with the same diameter.

During the drawing operation, the fiber 20 moves through a diametergauge 26 which produces on a lead 27 a signal representing the diameterof the fiber. That signal is forwarded to an input of a detection,analysis and feedback processor 28. A control signal produced by theprocessor 28 is carried by a lead 29 for controlling the temperature ofthe furnace 25.

Thereafter the fiber 20 moves along through an optional heater 30 andthermometer, or pyrometer, 31 for supplementing the residual heat in thefiber 20, if desired, and monitoring the temperature of the fiber 20.Temperature measured by the thermometer 31 is applied through a lead 32to the processor 28, which produces a signal on a lead 33 forcontrolling the temperature of the heater 30. In a variable length, ortelescoping, chamber 34, an exemplary mixture of acetylene precursor gastogether with chlorine and an inert gas, such as nitrogen, argon, orhelium, is applied to the hot moving surface of the fiber 20 forinducing decomposition of the acetylene precursor gas and depositing aconductive carbon coating uniformly around the periphery of the fiber.Chlorine is used as a getter for free hydrogen. Length of the chamber 34is transmitted via lead 60 to the processor 28. The coating depositionprocess occurs in controlled ambient conditions, which typically arestable. While the fiber is moving and without it contacting anyapparatus, the thickness of the exemplary conductive carbon coating iscontinuously controlled within tight tolerances to effectivelyhermetically seal the optical fiber 20 from any subsequent contact witheither water or hydrogen during fabrication, installation, or use of thefiber in a transmission system. At the same time, vulnerability of thecoating to cracking under tensile load due to excessive coatingthickness is limited to maintain an acceptable level of fiber strength.

Referring now to FIG. 2, there is shown a diagram of the bare opticalfiber 20 including a center core 22 and cladding 24. Although not shownin FIG. 2, there may be more layers than the core and single layer ofcladding for the bare optical fiber.

Referring now to FIG. 3, there is shown a diagram of the optical fiber20 coated with a thin layer of carbon 27, as represented by dotsoverall.

After the carbon coating is applied to the moving fiber 20 of FIG. 1,the coated fiber moves on through a radio frequency resonant cavity 35for measuring the thickness of the carbon coating. Radio frequency is anelectromagnetic wave frequency intermediate between audio frequency andinfrared frequency. A cavity in the microwave range of frequencies hasbeen used successfully because of component sizes and availability. Amicrowave is a very short wavelength electromagnetic wave, typicallyless than thirty centimeters in wavelength. Upon exiting from theresonant resonant frequency cavity 35, the fiber 20 moves on through oneor more vessels 46 which are filled with ultraviolet light curableliquid materials that are subsequently transformed into polymeric solidsfor jacketing the fiber 20 to protect its surface from future mechanicaldamage resulting from incidental or accidental contact. Thistransformation to the polymeric solid is made by a set of lamps 47applying ultraviolet light. Once the jacket is formed on the fiber 20,it is wound about a capstan drive 48 and then onto a reel 49 for storageand for convenience of handling until the fiber is installed in atransmission system. Speed of the capstan 48 and the fiber are sent tothe processor 28 via a lead 61.

Two exemplary subsystems control the previously described optical fiberdrawing operation. The first control system, including the diametergauge 26, determines the diameter of the fiber by a measurement made inan optical chamber and by analysis in the processor 28 converts suchmeasurement into an analogous control signal. This fiber diametercontrol signal is applied by way of the lead 29 to the furnace 25 foradjusting the furnace temperature and/or by way of a lead 54 to thecapstan drive control 55 for adjusting the drawing speed so that thefiber diameter is kept within predetermined tolerances. Controllingfiber diameter by operation of the furnace and the drawing speed aredescribed in detail in a textbook, entitled "Optical FiberTelecommunications" edited by S. E. Miller et al., Academic Press, Inc.,1979, pp. 263-298.

Another control system measures and controls the thickness of the carboncoating that is applied to the surface of the moving optical fiber 20without physically contacting the unjacketed fiber. This method formeasuring and continuously controlling the thickness of the carboncoating operates on one or more of the following parameters: fibertemperature, acetylene gas pressure, time the fiber is exposed toacetylene, or concentration of acetylene gas; and is an example of thenew method of the invention which is described in detail hereinafter.Pressure in the chamber 34 is transmitted by way of a lead 62 to theprocessor 28. A control signal for changing the pressure is sent fromthe processor 28 via a lead 56 to a pressure regulator 57. An indicationof the concentration of the acetylene gas is forwarded from the chamber34 by way of a lead 63 to the processor 28. A control signal or controlsignals for changing the mixture of gases is transmitted from theprocessor 28 via a lead 64 to gas supply valves 65, 66 and 67. The gasesare mixed in a manifold 68 and delivered through the pressure regulator57 and supply line 40 to the gas chamber 34. The gases exit the gaschamber 34 by way of an exhaust fitting 45. Although the cavity 35 isshown preceding the ultraviolet light curable jacketing supply vessels46, the cavity 35 could be located after the vessels 46 and the set oflamps 47.

Referring now to FIG. 4, there is shown a detailed perspective view ofthe resonant cavity 35. FIG. 4 shows an exemplary resonant cavity foroperation in the microwave frequency range. Shown in FIG. 4 is a sectionof hollow rectangular microwave waveguide 37, which has a length equalto a half guide wavelength at the operating frequency. Each end of thewaveguide section is shorted by one of the conducting plates 38 and 39.A microwave frequency input signal, produced by a fixed or sweptfrequency signal generator 41, is coupled through a coaxial line 42, aconnector 43, and an opening through the shorting plate 38 to a loop 44of the center conductor to ground. This magnetic input loop ispositioned inside of the resonant cavity to energize a resonantelectromagnetic field in response to the applied input signal. Powerlevels of the input signal typically can be in the range of a fractionof a milliwatt to about 100 mw.

According to the standard adopted for FIG. 4, the array of arrowsrepresent the electric field vectors in the resonant cavity atresonance. These electric field vectors present an instantaneouscondition of a continuously alternating electric field. Longitudinallyfrom the input, the amplitude of the electric field represents astanding wave that increases from zero at the input shorting plate 38 toa maximum amplitude at the center of the cavity. As shown on an axisalong the bottom of the cavity, there is shown a scale marking off ahalf guide wavelength λ_(g) /2 between the input shorting plate 38 andthe output shorting plate 39. The amplitude of the electric fielddecreases to zero at the output shorting plate 39. Although the lengthof waveguide section 37 is shown as one half guide wavelength at theoperating frequency, other multiples of a half guide wavelength can beused.

At the center cross-section of the cavity, there are shown twointersecting planes of arrows representing the electric field for thestanding wave. This electric field has a maximum amplitude at or nearthe center of the cavity and an amplitude decreasing to zero at bothsides of the waveguide.

A coated optical fiber moves through openings cut into the top andbottom walls of the section of waveguide. These openings are positionedopposite each other so that the optical fiber is readily threadedtherethrough and moves continuously without physically contacting thewaveguide structure. The openings are placed where the electric fieldhas sufficient strength to provide a useful signal and is sufficientlyuniform across the opening to make convenient measurements on the movingfiber during the fiber coating operation.

Although it is not shown in FIG. 4, there is a magnetic field whichexists concurrently with the electric field. Such magnetic field isdirected at a right angle with respect to the direction of the electricfield. Together the electric field and the magnetic field make up anelectromagnetic field within the resonant cavity in response to theenergizing input signal.

An output signal can be extracted from the resonant cavity by either anelectric field probe or a magnetic field probe. In either case the probeis positioned so that it interacts with the appropriate field where thefield strength is sufficient to produce a useful output signal.

As an example in FIG. 4, consider that an output signal is extractedfrom the cavity by an output magnetic field loop 50 that is insertedinto the cavity through the output shorting plate 39. This outputcoupling loop is formed, e.g., by bending the center conductor of acoaxial connector 53 to the output shorting plate 39. Coaxial line 52transmits output signals from the output coupling loop 50 and theconnector 53 to a detection, analysis and feedback processor 28.

It is important to emphasize that although the energy insertion, orinput, coupling loop 44 and the energy extraction, or output, couplingloop 50 are shown as magnetic field probes positioned for optimummagnetic field strength, they may be replaced by input and outputelectric field probes appropriately positioned for an optimum electricfield. As an alternative to the coaxial probe arrangement, otherwaveguide transmission media can be substituted for the connectors 43and 53 by coupling through appropriate holes in the shorting plates 38and 39.

Referring now to FIG. 5, there is shown a graph of vectors representingthe electromagnetic energy at the shorting plate 39 of FIG. 4. At theleft of the origin of the three dimensional x, y, z axis, there are anincident electric field vector E_(i), an incident magnetic field vectorH_(i), and a velocity vector γ_(i) for the incident electromagneticwave. It is noted that the electric field vector is oriented in thepositive vertical direction and that the magnetic field vector isoriented horizontally on the three dimensional axis. When the incidentwave reaches the output shorting plate 39 located at the origin of thehorizontal axis, a reflected wave is produced. Since there is a shortcircuit, the effective voltage is zero and the reflected electric fieldE_(r) is oriented in the negative vertical direction. Its magnitudeequals the magnitude of the incident electric field E_(i).

At the same location, the incident magnetic field H_(i) also isreflected. For the reflected magnetic field H_(r), polarity is the sameas the incident magnetic field H_(i), but amplitude is doubled becauseof the output shorting plate 39. Also the reflected velocity vectorγ_(r) is reversed in direction from the incident velocity vector γ_(i).

Exemplary input and output magnetic coupling loops 44 and 50 of FIG. 4are oriented to couple energy into and extract energy from the resonantcavity at or near the points of maximum magnetic field or at points withsufficient magnetic field strength to produce a useful signal at theoutput coupling loop 50.

The exemplary detection, analysis and feedback processor 28 of FIGS. 1and 4 performs multiple functions. Among those functions are: (1) theproduction of an output signal power versus frequency characteristiccurve for the resonant cavity 35, (2) comparing the output signal powerversus frequency characteristic curve to a reference characteristiccurve, (3) determining the difference between the output signalcharacteristic and the reference characteristic curve, (4) by way of alead 56 sending a control signal related to that difference to a controlelement 57 of FIG. 1 for changing the flow of or the partial pressure ofacetylene precursor gas, (5) by way of the lead 54 for changing thespeed of the drawing operation to control the fiber temperature as itenters the acetylene chamber, (6) by way of a lead 58 for changing thelength of the chamber 34 to control the time of exposure to theacetylene, and (7) by way of the lead 33 for changing the temperature ofthe fiber and the coating on the fiber. The adjustments of theseaforementioned coating process parameters are made for changing the rateat which the carbon coating is deposited on the moving fiber. To controlany specific coating process, one or more of these parameters iscontrolled.

Referring now to FIG. 6, there is shown a plot of the output energyversus frequency response curve for three different resonant conditionsof the cavity 35 of FIG. 4. In FIG. 6 a narrow, spiked response curve 76represents the output signal energy versus frequency for the emptycavity 35 or for the cavity 35 with an uncoated optical fiber 20inserted therein. Since the optical fiber 20 is fabricated basically insilica, a dielectric, the fiber is an electric insulator, ornonconductor, and causes very little effect on the empty cavity responsecurve. Response curve 76 is a useful reference for control purposes.

Two other response curves 78 and 80 represent curves of the desiredupper and lower limits on the thickness of the carbon coating which isdeposited on the optical fiber 20 of FIG. 4. Because carbon isconductive and because the coating is substantially aligned with theelectric field in the cavity, that field induces a current in an axialdirection along the conductive carbon coating on the fiber. Conductivityis a number proportioned to the current from one face of a unit cube ofthe coating material to the opposite face of that cube when a unitpotential difference is maintained between the two faces. Being aconductor, the carbon coating is capable of carrying an electriccurrent. Such a current alternates in direction at the frequency of theapplied electric field and creates a magnetic field around the fiber 20.This action distorts the electromagnetic field in the cavity anddissipates or absorbs power. As a result, the output energy versusfrequency curve is reduced in amplitude and is spread out from the shapeof the narrow empty cavity response curve 76, and the resonant frequencychanges. A continuous family of curves results. Only, three curves ofthe family are shown. By calibrating the resulting response curves forthe desired range of conductance and therefore carbon thickness, theoutput response curves 78 and 80 and others of the family aresubsequently useful with reference to the curve 76 for measuring thethickness of the carbon coating as it is deposited on the optical fiber20. Typically conductance is a ratio of the current carried by thecoating to the applied electromotive force (reciprocal of resistance) atd.c. For our purpose, we are measuring an effective radio frequencyconductance.

The detection, analysis and feedback processor 28 analyzes the outputenergy versus frequency data continuously during a fiber drawingoperation. From the values of amplitude and frequency in comparison withthose of the reference curve, the processor 28 determines both thequality factor Q of the cavity and the conductance of the coating. Sucha conductance determination or measurement is readily convertible tothickness data of the coating and to a determination of whether or notthe thickness data is within the desired limits. As a result of theprocessing, the processor 28 generates a signal which when fedback tocontrol the coating process maintains the carbon coating thicknesswithin the desired limits by controlling one or more of the processparameters: fiber temperature, precursor gas density, fiber exposuretime, or precursor gas pressure.

Referring now to FIG. 7, there is shown another configuration of aresonant cavity 90. Except for the round cylindrical shape of the cavity90, the arrangement and operation of the detection, analysis andfeedback processor of FIG. 7 are similar to the apparatus of FIG. 4. Theoptical fiber 20, coated with carbon, moves through the cavity 90 in apath so that the coating is aligned with the energized electric field inthe cavity. For the fiber 20, entrance and exit openings in the cavity90 are located at or near the position of sufficient electric fieldstrength to produce a detectable output signal. Thickness of the carbonis controlled by a feedback control signal, as described previously.

Referring now to FIG. 8, there is shown another arrangement fordetermining the thickness of the carbon coating on the moving opticalfiber. In FIG. 8 there are two round cylindrical cavities 110 and 112with an intercoupling section 115. When the signal source 41 applies analternating current signal through the line 42 and the input couplingloop 44 to the cavity 110, an electromagnetic field is establishedtherein. Some of the energy from the electromagnetic field in the firstcavity 110 is coupled through the coupling section 115 into the secondcavity 112. From the second cavity, output signal energy is coupled intothe output coupling loop 50 and is transmitted to the detection,analysis and feedback processor 28.

The carbon coated optical fiber 20 moves through an entrance openinginto the second cavity 112, the coupling section 115 and the firstcavity 110 and out from an exit opening. Thickness of the carbon coatingis determined by the same procedure as previously described. Thicknessis measured by the processor 28 which also determines a control signalthat controls the carbon depositing operation, as illustrated in FIG. 1.

Referring now to FIG. 9, there is shown a carbon coated optical fibermoving through longitudinal slots 130 and 132 in a section of waveguide135. From the source 41 and the line 42, the waveguide is energized witha propagating electromagnetic field. Slots 130 and 132 are cut in thecenterlines of the opposite broad faces of the section of waveguide. Theslots should be as narrow as reasonably possible consistent with thefiber never contacting the slot boundaries.

For convenience of available parts and the sizes of those parts, asection of WR90 waveguide and a klystron oscillating at 10.5 GHz werechosen for the arrangement. A crystal diode is used for the detector atthe output end of the waveguide 135. Matching sections are use forconnecting the klystron and the detector to the section of waveguide. Atlow incident power levels, e.g., less than 100 microwatts, the outputcurrent of the detector is proportional to the square of the electricfield in the waveguide. That output current is therefore proportional tooutput power. The foregoing specific items and parameters are mentionedby way of example. Other sizes, frequencies and power levels also areuseful.

As previously mentioned, the input signal, applied by the klystron inFIG. 9, creates a propagating electromagnetic field in the section ofwaveguide. In the unlikely event that the arrangement is perfectlymatched, the magnitude of the time averaged vertical electric field isconstant along the entire axis of the section of waveguide 135 in theabsence of the coated fiber. In that case the longitudinal position ofthe fiber is not important. In the more likely event that thearrangement is mismatched, there are standing waves present andtherefore at least a partially resonant condition in the section ofwaveguide 135. In the mismatched arrangement, there are positions ofgreater electric field strength where more power is absorbed by theconductive coating. A deliberate mismatch can be created by tuningscrews and/or irises in the waveguide. In this mismatched case, thefiber is deliberately located in a longitudinal position coincident withan electric field maximum which corresponds to a minimum longitudinalfield gradient.

When the fiber 20, with the conductive carbon coating, moves through thewaveguide, the conductive coating will interact with the electric fieldcomponent which is parallel with the axis of the fiber. Transmittedpower is reduced by the creation of an alternating current in the carboncoating. The amount of power lost is a function of the conductance ofthe carbon coating on the fiber. For a uniform outside diameter opticalfiber, conductance of the carbon coating depends upon the conductivityof the carbon and the carbon coating thickness, which is a variable.

Thus depending upon the variable thickness of the carbon coating, thedetection and analysis processor 28 will measure a variable transmissionloss for a specific constant input signal power at a given frequency.

FIG. 10 shows the result of measurements 134, 136, 137, 138 and 139 oftransmitted power loss versus conductance or thickness of some practicalexamples of carbon coatings.

Referring now to FIG. 11, there is shown a section of a coaxialtransmission line 140 having an outer cylinder conductor 141 and acenter conductor 142 separated by a dielectric 144. When this coaxialtransmission line 140 is energized by an alternating current signalsource 41, an electromagnetic field is established along its length.Detection, analysis and feedback processor 28 determines the magnitudeof the output signal at the far end of the section of coaxialtransmission line 140. Along a selected diameter of the coaxialtransmission line 140, there is a hole 146 cut all of the way throughthe coaxial transmission line. The optical fiber 20 to be measured movesthrough the hole 146 during the fiber drawing operation.

Since the energized electromagnetic field establishes a radial electricfield, as shown in the cross section of FIG. 12, there are components ofthat electric field which are oriented more or less parallel with thecoating on the fiber 20, as shown in FIGS. 11 and 12. The conductivecarbon coating will have an alternating current induced by thecomponents of the radial electric field. Thus power is absorbed ordissipated by the carbon coated fiber, and the resulting reduction ofoutput signal power is measured by the detection, analysis and feedbackprocessor 28.

Referring now to FIG. 13, there is shown a coaxial resonator arrangement150 for measuring and controlling the thickness of the carbon coating onthe optical fiber 20. An outer concentric conductor 151 and a centerconductor 152 are separated by a dielectric 154. Shorting plates 155 and156 are affixed to each end of the coaxial resonator 150. A center hole157 is cut through the entire transmission line arrangement along thecenter axis of the center conductor 152, as shown in FIG. 14. Thecoaxial resonator arrangement 150 of FIG. 13 is energized by a source ofalternating current signals 41 to create an electromagnetic fieldindicated by arrows in the dielectric 154.

During a drawing operation for the fiber 20, the portion of the electricfield which is aligned with the axis of the fiber 20 induces analternating current in the conductive carbon coating on the movingfiber. Detection, analysis and feedback processor 28 measures thethickness of the carbon coating and produces a signal which controls thecarbon deposition process.

In FIG. 15 the measuring arrangement 170 includes a conductive chamber171 and a conductive coil 172, affixed only at one end 173 to theconductive chamber 171 and generally separated from the chamber by adielectric 174, such as air. The coil 172 can be fabricated from asuperconductor material. By energizing the coil 172 with a radiofrequency signal from a source 41 using an input coupling loop 176, anelectromagnetic field is established within and along the coil 172. Withproper design of the arrangement 170, the alternating electric fieldcomponent is strong along the center axis of the coil 172, as shown bythe arrows directed along the center axis. The carbon coated fiber 20moves through the electric field and absorbs power from it. Detectedpower is extracted from the resonator by way of an output coupling loop177. Electrical coupling can be substituted for the input and/or outputmagnetic coupling loops. The transmission response is related toconductance of the carbon coating. Detection, analysis and feedbackprocessor 28 determines the thickness of the conductive carbon coatingand develops a signal for controlling the carbon deposition process.

In FIG. 16 the measuring arrangement 180 includes a circular electricTE₀₁ mode resonant cavity 181 operated in the millimeter wavelength bandso as to develop a circumferential electric field of sufficientmagnitude at the surface of the optical fiber 20 to permit significantinteraction between the conductive coating around the periphery of thefiber and the resonant cavity behavior. Here to assure TE₀₁ modeoperation, the cylindrical wall 183 can be an anisotropic conductor,i.e., one that favors circumferential wall current flow anddiscriminates against axial wall current flow, such as described by S.E. Miller in U.S. Pat. No. 2,848,696. Other higher order circularelectric modes can also be used in the cavity.

In order to assure efficient launching and detection of the TE₀₁ modeand discrimination against other unwanted modes, antiphased magneticprobes--in the form of small waveguide coupling holes 184--have beenintroduced into the waveguide cavity short circuit end plates 185 and186. The antiphase arrangement in this example is illustrated by feedinga microwave source 41 into the difference port Δ of a hybrid junctionand waveguides 188 and by extracting output signals from the cavity viathe waveguides 189 and another hybrid. The sum ports Σ are terminated bytermination elements 187.

As in the prior examples discussed, the resonant cavity behavior is thatshown in FIG. 6. Output energy is coupled through the difference port Δand the lead 52 to the detection, analysis and feedback processor 28 formeasuring and controlling the thickness of the coating on the opticalfiber 20.

To minimize undesirable effects caused by ambient changes, we have usedseveral techniques such as: practicing the fiber drawing process and thecoating process in a stable controlled environment; using a highlystable oscillator in a temperature controlled environment for thepropagating electromagnetic field measurement set up on FIG. 9; using aPound stabilizer, such as described by T. H. Wilmshurst, "Electron SpinResonance Spectrometers", Plenum Press, New York, pages 199-204; using astable power supply; using a mechanical configuration which minimizesdistortion of the waveguide; using coaxial cable sections that produceno more than a small phase shift on flexing; using low thermal expansionwaveguide sections; using minimum length microwave paths; and using atwo arm system fed by a common source including two identical waveguidesensor systems--one as a reference, the other for measuring the effectof the coated fiber on the output power. Whether these techniques areused or not depends upon the sensitivity required and the severity ofambient changes in the test environment.

All of the previously described two port devices have single portanalogs that can utilize reflectometers or circulators to accomplish thesame functions.

Dynamic control of the thickness of the coating deposited on the opticalfiber is achieved by sensing and adjusting one or more of the followingfour parameters: (1) the temperature of the fiber entering the precursorgas chamber; (2) in the precursor gas chamber, the concentration of theacetylene gas including the carbon atoms to be deposited; (3) theacetylene gas pressure in the precursor gas chamber; and (4) the time ofexposure of the hot fiber to the acetylene gas in the chamber. Uponclose analysis, other arrangements which might: (1) move the position ofthe precursor gas chamber; (2) change the length of the precursor gaschamber; (3) change the mixture of the gases; (4) vary the speed of thefiber draw; or (5) alter the temperature of the furnace-fundamentallyalter one or more of the four described parameters of the manufacturingprocess.

Thus there has been described a method for measuring and dynamicallycontrolling the thickness of a thin carbon coating deposited on a movingoptical fiber. None of the arrangements for performing these methods ofmeasuring and controlling physically contacts the fiber or the coatingduring the manufacturing operation. All of the described methodstogether with other methods made obvious in view thereof are consideredto be covered by the appended claims.

We claim:
 1. A method for determining the thickness of a conductivecoating on an elongated body of a dielectric material, which includesthe steps of(a) establishing an electromagnetic field in a section of ahollow metallic waveguide,(i) said waveguide section being terminated bymatched terminations on opposite ends thereof, (ii) said electromagneticfield being established by said matched terminations, and having afrequency of oscillation in the radio frequency range of from 10 MHz to150 GHz, (iii) said waveguide section having an opening in each of theopposite broad walls thereof permitting passage of an elongated bodytransverse of the waveguide section in substantial alignment with theelectric field component of the electromagnetic field and without anyphysical contact with the walls of the waveguide section, (b) moving aconductively coated elongated dielectric body through said openings sothat it passes through the electromagnetic field, and (c) in response toa change in the electromagnetic field, generating a signalrepresentative of the thickness of the conductive coating.
 2. The methodof claim 1, whereinthe elongated body with the conductive coating movingthrough the waveguide perturbs the electromagnetic field by conductingcurrent and dissipating power.
 3. The method of claim 1, comprising thefollowing steps:applying an input signal to establish theelectromagnetic field in an empty waveguide; extracting a first energysample from the electromagnetic field in the empty waveguide; moving theconductively coated elongated body through the waveguide with thecoating in substantial alignment with an electric field of theelectromagnetic field; extracting a second energy sample from thewaveguide including the coated elongated body moving through thewaveguide with the coating in substantial alignment with the electricfield; and comparing the second energy sample with the first energysample to determine the thickness of the coating.
 4. The method of claim1, in which said frequency of oscillation is about 10.5 GHz.
 5. Themethod of claim 1, in which said elongated body is an optical fiber andsaid conductive coating is carbon.